What is multiple re-inheritance?

Question

I refer to the following as “multiple re-inheritance”:

• inheriting a class once directly and one or more times indirectly by inheriting one or more of its descendan

Answer 1

The common term for this is the diamond pattern (or diamond problem).

It is not an error per se, but as noted in the comments here, any attempt to access a direct base that is reduplicated elsewhere in the hierarchy will result in an ambiguity error.

One workaround would be to make the base indirect. The new inheriting constructors feature in C++11 allows perfect wrappers:

template< typename base, typename tag >
struct disambiguated_base : base
    { using base::base; };

Given an unused tag type, this generates a new class derived from, and functionally identical to, the given base. The tag type may be an incomplete class denoted by an elaborated-type-specifier:

struct GrandChild : Parent,
    disambiguated_base< GrandParent, class grandchild_grandparent_tag > {

    typedef disambiguated_base< GrandParent, grandchild_grandparent_tag >
        my_direct_grandparent;

    int num;
};

Now GrandChild can use my_direct_grandparent:: to disambiguate member accesses.